This application claims the benefit of Korean Patent Application No. 1999-53712, filed on Nov. 30, 2000, which is hereby incorporated by reference for all purposes as if fully set forth herein.
1. Field of the Invention
The present invention relates to X-ray image sensors. More particularly, it relates to X-ray image sensors having a TFT (Thin Film Transistor) array, and to a method for fabricating the same.
2. Discussion of the Related Art
X-ray detection has been widely used for medical diagnosis. X-ray detection typically uses an X-ray film to produce a photograph. Therefore, some predetermined developing and printing procedures are required to produce the photograph.
However, digital X-ray image sensors that employ TFTs (Thin Film Transistors) have been developed. Such X-ray image sensors have the advantage that real time diagnosis can be obtained.
FIG. 1 is a schematic, cross-sectional view illustrating the structure and operation of an X-ray image sensing device 100. Included are a lower substrate 1, a thin film transistor 3, a storage capacitor 10, a pixel electrode 12, a photoconductive film 2, a protection film 20, a conductive electrode 24 and a high voltage D.C. (direct current) power supply 26.
The photoconductive film 2 produces electron-hole pairs in proportion to the strength of external signals (such as incident electromagnetic waves or magnetic waves). That is, the photoconductive film 2 acts as a converter that converts external signals, particularly X-rays, into electric signals. Either the electrons or the holes are then gathered by the pixel electrode 12 as charges. The pixel electrode is located beneath the photoconductive film 2. Which charge species that is gathered depends on the voltage (Ev) polarity that is applied to the conductive electrode 24 by the high voltage D.C. power supply 26. The gathered species charges are accumulated in the storage capacitor 10, which is formed in connection with a grounding line. Charges in the storage capacitor 10 are then selectively transferred through the TFT 3, which is controlled externally, to an external image display device that forms an X-ray image.
In such an X-ray image sensing device, to detect and convert weak X-ray signals into electric charges it is beneficial to decrease the trap state density (for the electric charge) in the photoconductive film 2, and to decrease charge flow in non-vertical directions. Decreasing non-vertical charge flow is usually accomplished by applying a relatively high voltage between the conductive electrode 24 and the pixel electrode 12.
Electric charges in the photoconductive film 2 are trapped and gathered not only on the pixel electrode 12, but also over the channel region of the TFT 3. Even during the OFF state, the electric charges trapped and gathered on the pixel electrode 12 and on the channel region of the TFT 3 induce a potential difference between the TFT 3 and the pixel electrode. This has a similar effect as the TFT 3 being in the ON state. This adversely affects the switching of the TFT 3 and increases the OFF state leakage current. Such can result in an undesired image.
FIG. 2 is a plan view illustrating one pixel 102 of the X-ray image sensor panel 100. Shown are the TFT 3, a storage capacitor xe2x80x9cSxe2x80x9d and a pixel electrode 62 that collects charges.
The TFT 3 includes a gate electrode 31, which is formed by an elongation of a gate line 50, and a drain electrode 32, which is formed by an elongation of a drain line 52.
The storage capacitor xe2x80x9cSxe2x80x9d is comprised of transparent first and second capacitor electrodes 58 and 60. A ground line 42 acts as a common electrode that is shared by adjacent pixels. Also shown are first contact holes 54 that connects the pixel electrode 62 with a source electrode 33 of the TFT 3, and a second contact hole 56 that connects the pixel electrode 62 with the second capacitor electrode 60.
According to the conventional art, an X-ray image sensor includes a photoelectric conversion part that produces electric charges in accordance with received electromagnetic energy; a charge storage capacitor xe2x80x9cSxe2x80x9d having a first capacitor electrode 58, a dielectric layer that is deposited on the first capacitor electrode 58, a second capacitor electrode 60 on the dielectric layer, a protection film having multiple contact hole(s) 54 and 56 on the second capacitor electrode 60, and a pixel electrode 62 that is formed on the protection film. The pixel electrode is in contact with the second capacitor electrode 60 through the contact hole(s) 56 and collects the electric charges produced in the photoelectric conversion part. A switching TFT 3 controls the release of the electric charges stored in the storage capacitor xe2x80x9cSxe2x80x9d. The switching TFT includes a gate electrode 31, a drain electrode 32, and a source electrode 33 that contacts the pixel electrode 62.
FIGS. 3a to 3f are sectional views, taken along the line IIIxe2x80x94III of FIG. 2, that illustrate a manufacturing process.
Referring to FIG. 3a, a metal layer is deposited and patterned on a substrate 1 to form a taper-shaped gate electrode 31. The substrate 1 can be a quartz substrate or a glass substrate. However, the substrate 1 is beneficially a glass panel since quartz panels are relatively expensive. The gate electrode 31 can made of a metallic material selected from a group comprised of Molybdenum (Mo), Tantalum (Ta), Tungsten (W), Niobium (Nb), and Antimony (Sb).
FIG. 3b illustrates the steps of depositing a first insulation film 102 and a semiconductor layer 104. The gate insulation film 102 is formed by a deposition of an inorganic insulation film (such as a silicon nitride (SiNx) film or a silicon oxide(SiOx) film) having 4000 xc3x85 thickness. Alternatively, an organic insulation material such as BCB (benzocyclobutene) or acrylic resin can be used. After the deposition of the first insulation film 102, a dual layer semiconductor film 104 comprised of an amorphous silicon layer 104a and a doped amorphous silicon film 104b are deposited. Although vapor deposition or ion injection can be used for the formation of the doped amorphous silicon film 104b, vapor deposition is usually employed.
Next, as shown FIG. 3c, a second metal layer is deposited for both the source electrode 33 and the drain electrode 32, and for the sound line 42. That metal, beneficially Chromium (Cr) or a Cr-alloy, is then patterned to form the source electrode 33, the drain electrode 32 and the ground line 42. Moreover, the portion of the doped amorphous silicon film 104b between the source and drain electrodes 33 and 32 is eliminated by using the source and drain electrodes as masks. Then, a first capacitor electrode 58 is formed over the ground line 42. The first capacitor electrode 58 is beneficially comprised of a transparent electrode material such as ITO (indium tin oxide). The region C in FIG. 3c designates a switching transistor.
Referring to FIG. 3d, a silicon nitride film having a thickness of 3000 xc3x85 forms a second insulation film 106 is deposited on the source and drain electrodes 33 and 32, and on the first capacitor electrode 58. The second insulating film 106 acts as protective layer for the TFT 3 and as a dielectric for a capacitor that is being formed with the first capacitor electrode 58.
After the second insulation film 106 is deposited a second capacitor electrode 60 is formed on the second insulation film 105 and over the first capacitor electrode 58. Beneficially, the second capacitor electrode is the same size as or a little larger than the first capacitor electrode 58.
As shown in FIG. 3e, an insulating protection film 108 is then formed. An organic substance such as BCB (benzocyclobutene) is beneficially used. BCB is a material that has a lower dielectric constant than silicon nitride, silicon oxide or acrylic resin. After formation of the protection film 108, first and second contact holes 54 and 56 are formed through the protection film 108. The first contact hole 54 exposes a portion of the source electrode 33. The second contact hole 56 exposes a portion of the second capacitor electrode 60. Although the first contact hole 54 penetrates down to the source electrode 33, the second contact hole 56 can not go as deep since the second capacitor electrode 60 acts as an etch stop that prevents the second insulation film 106 from being etched.
FIG. 3f illustrates the step of forming a pixel electrode 62 (a third transparent electrode layer). The pixel electrode is formed over the second insulation film 106 such that the pixel electrode extends into the first and second contact holes 54 and 56 and electrically connects with the source electrode and the second capacitor electrode 60. In addition, the pixel electrode 62 is formed such that it extends over the TFT 3.
The next step is the application of a light-sensitive material 123. That material converts received external signals (X-rays) into electric charges. The light-sensitive material 123 is beneficially comprised of an amorphous selenium compound that is deposited in a thickness of 100 to 500 xcexcm by an evaporator. However, other X-ray-sensitive materials that having low dark conductivity and high sensitivity to external signals, for example HgI2, PbO2, CdTe, CdSe Thallium bromide or cadmium sulfide can also be used. When the light-sensitive material is exposed to X-rays, electron-hole pairs are produced in the light-sensitive material in accordance with the strength of the x-rays.
After the application of the X-ray-sensitive material, a transparent conductive electrode 133 that passes X-ray is formed. When a voltage is applied to the transparent conductive electrode 133 while X-rays arc being irradiated electron-hole pairs formed in the light-sensitive material are separated into charges that are gathered to the pixel electrode 62 and stored in the storage capacitor xe2x80x9cSxe2x80x9d.
According to the mentioned conventional X-ray image sensing device, however, the depositing and patterning of the electrodes are performed three times to fabricating the storage capacitor xe2x80x9cSxe2x80x9d. Moreover, the gate line, and the source and drain electrode are overetched while etching the ITO layers.
Moreover, as shown FIG. 4, due to the shortness of the length xe2x80x9cxcex94Lxe2x80x9d between the first capacitor electrode 58 and the source electrode 33, a short-circuit can result.
Furthermore, a parasitic capacitor between the drain line 52 and the first capacitor electrode 58 can cause problems.
The present invention has been developed as a result of continuous effort by the inventors to solve the above-described problems.
Accordingly, the present invention is directed to an x-ray image sensor and to a method for fabricating the same and that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide an X-ray image sensor having simpler processing steps while forming ITO (indium tin oxide) electrodes.
Another object of the present invention is to provide an X-ray image sensor having improved yields.
A further object of the invention is to provide a method of forming an X-ray image sensor which can reduce processing error during production by preventing short-circuits and which can decrease noise due to a parasitic capacitor.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve the above objects, the present invention provides an X-ray image sensor, including: a substrate; a gate electrode on the substrate; a first insulation fill on the substrate that covers the gate electrode; a semiconductor film of amorphous silicon film on the first insulation film over the gate electrode; a doped amorphous silicon film on the semiconductor film; source and drain electrodes on the doped amorphous silicon film that are spaced apart from each other; a ground line on the first insulation film that is spaced apart from the source and gate electrodes; a second insulation film covering the whole substrate and having first and second contact holes that expose portions of the ground line and the source electrode, respectively; a first capacitor electrode on the second insulation film, the first capacitor electrode having an electrical connection with the ground line through the first contact hole; an electron transport electrode on the second insulation film, the electron transport electrode having an electrical connection with the source electrode through the second contact hole; a dielectric layer covering the second insulation film, the first capacitor electrode and the electron transport electrode, the dielectric layer having a third contact hole; and a pixel electrode on the dielectric layer having an electrical connection with the electron transport electrode.
Beneficially, the pixel electrode extends over the semiconductor film, and the insulation layers are made of a material selected from the group consisting of BCB (benzocyclobutene), acryl and polyamide. Moreover, the X-ray image sensor includes a light-sensitive material on the pixel electrode.
In order to achieve the above objects, the invention also provides a method for fabricating an X-ray image sensor including: providing a substrate; forming a gate electrode on the substrate; forming a first insulation film on the substrate and that covers the gate electrode; forming a semiconductor film of amorphous silicon film on the first insulation film over the gate electrode; forming a doped amorphous silicon film on the semiconductor film; forming source and drain electrodes on the doped amorphous silicon film that are spaced apart from each other, forming a channel region by eliminating the doped amorphous silicon film between the source and drain electrodes by using the source and drain electrodes as a mask; forming a ground line on the first insulation film that is spaced apart from the source and gate electrodes; forming a second insulation film covering the whole substrate; forming first and second contact holes that expose a portion of the ground line and a portion of the source electrode, respectively; forming a first capacitor electrode on the second insulation film, the first capacitor electrode having an electrical connection with the ground line through the first contact hole; forming an electron transport electrode on the second insulation film, the electron transport electrode having an electrical connection with the source electrode through the second contact hole; forming a dielectric layer that covers the second insulation film, the first capacitor electrode and the electron transport electrode; forming a third contact hole in the dielectric layer; forming a pixel electron on the dielectric layer, the pixel electrode having an electrical connection with the electron transport electrode; forming a light-sensitive material on the pixel electrode; and forming a transparent conductive electrode that passes X-rays on the light-sensitive material.
The insulation layers are beneficially made of a material selected from the group consisting of BCB (benzocyclobutene), acryl and polyamide. The capacitor electrode and the pixel electrode are beneficially made of transparent ITO (indium tin oxide). The X-ray-sensitive material is beneficially one of a group consisting of HgI2, PbO2, CdTe, CdSe, Thallium Bromide, and Cadmium Sulfide etc.
It is to be understood that both toe foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.